Sorting systems for paper production serve to separate a fiber suspension into at least two fractions, namely into a so-called fine fraction and a so-called coarse fraction, with the fine fraction consisting in large part of the water contained in the fiber suspension and of as many paper fibers as possible, while the coarse fraction, i.e. the fraction which cannot pass through the screens used in the respective sorters of the sorting system, should contain as few fibers as possible and, where possible, all disturbing impurities.
Since the disturbing impurities to be removed have a wide size spectrum, it is unavoidable for the impurities formed by smaller and very small particles to enter into the fine fraction together with the fibers. To minimize the portion of impurities in the fine fraction and to prevent as much as possible that disturbing substances are present at all in the fine fraction obtained at the output of a sorting plant, complex and/or expensive sorting methods have been developed which require plants with a larger number of sorters which can be connected in series and/or in parallel. However, it has been found that the success of a sorting plant is not only determined by the number of sorting units used and by their quality, but also and above all by the technical process design of the sorting method itself.
With every high quality sorting method, the largest possible purity of the fine fraction obtained at the end, the lowest possible fiber loss, i.e. minimum fiber portions in the coarse fraction, and the largest possible production volume are aimed for, with production or production volume being understood as the obtained accepted stock.
A particular problem area in connection with the obtaining of this objective results above all from fluctuations in the raw material quality which can be caused, in a negative sense, by larger amounts of advertising flyers inserted into newspapers and, in a positive sense, by falling raw material prices, which promotes the processing of materials which result in an above-average raw material quality. Such states of affairs make it difficult to control or regulate sorting systems of a known kind such that a specific target parameter—such as efficiency or minimum fiber loss—is achieved.